This invention relates to a radar mapping technique employing a synthetic aperture radar which permits continuous mapping of points on the ground towards which an object moving in mid air is traveling in a rotating frame of reference or curvilinear path. Coherent side-looking imaging radars have been used to produce radar maps with photographic quality. High-resolution maps in two dimensions have been produced by an airborne radar through the generation of a synthetic aperture in the azimuth direction and pulse compression in the range direction. In the general operation of a side-looking radar, an airborne vehicle travels parallel to the ground with a given velocity. On board the vehicle is a pulse-doppler radar system whose physical aperture having a pre-determined beamwidth illuminates a ground-swath parallel to the flight path. The radar beam points in a direction which is orthogonal to the vehicle's flight path. The synthetic aperture radar system includes a transmitter from which range information is obtained by pulsing the transmitter. Fine range resolution and fine azimuth resolution are derived by storing the returned pulses over the time a ground scatterer stays within the physical aperture beamwidth, and then the stored data is processed in a correlator. Each pulse is linearly frequency modulated within each pulse by the transmitter, and the sequence of pulses, gathered over a beam dwell time, are doppler frequency modulated due to the airborne vehicle's path across the ground scatterer. The finely resolved images are displayed on a two dimensional display to form a radar map of the ground. On this display, the range coordinate is in the "cross-track" direction and azimuth is in the "along-track" direction. Thus, a physical aperture, whose dimensions are comensurate with the size of a spacecraft, in combination with a coherent radar and data processor can synthesize a very long antenna which is required to achieve a narrow beam in the azimuth dimension.
However, all such side-looking synthetic aperture radar systems have operated on the concept that all points located on the ground are in focus because the airborne vehicle is traveling in a path parallel to the ground instead of directly toward it. The radar mapping technique of the present invention utilizes a synthetic aperture radar to achieve continuous mapping of points on the ground towards which an object moving in mid air is traveling in a rotating frame of reference or curvilinear path. The technique of the present invention recognizes that the distortion phenomenon which results from traveling toward a desired point may be used for mapping the desired point for the purpose of directing a moving object toward that point.